In the commercial "wet process" production of orthophosphoric acid (H.sub.3 PO.sub.4), phosphate rock ("phosphorite"), which has the nominal chemical formula of Ca.sub.10 F.sub.2 (PO.sub.4).sub.6, is reacted in a aqueous system with strong sulfuric acid. The acid reaction results in the formation of calcium sulfate, orthophosphoric acid and hydrogen fluoride. The hydrogen fluoride is largely evolved in gaseous form, although some may remain ionized in the aqueous medium to form hydrofluoric acid. The calcium sulfate is then filtered off and the orthophosphoric acid is recovered by evaporation of the liquid and crystallization of the acid. In addition to removing the calcium sulfate product, the filtration step also serves to remove salts formed from oxide impurities which are in the phosphorite. These include the salts of magnesium, aluminum, iron, titanium and various other metals. The phosphorite is described in Ladoo et al, Nonmetallic Minerals (2d edn., 1951) on pages 379-393, and the orthophosphoric acid production reaction is described in Shreve et al, Chemical Process Industries (4th edn., 1977) on pages 252 and 253.
The filtration step is normally carried out using a rotary vacuum precoat filter. Dry diatomaceous earth ("diatomite") filter aid is used to form a precoat on the filter and is also used as a body feed in the filtration slurry system. The precoating and subsequent filtration using the aqueous acid system are carried out at temperatures on the order of 160.degree. F. (71.degree. C.) and higher. These temperature requirements, however, create difficulties in the filtration system both at the precoat stage and at the subsequent filtration stage using diatomite body feed, for diatomite does not disperse easily or completely in the orthophosphoric acid system at temperatures above 140.degree. F. (60.degree. C.). Thus, in order to obtain the desired benefits of a diatomite filtration system, the orthophosphoric acid producer must devote an inordinate amount of time to forming the diatomite precoat and thereafter to dispersing the diatomite body feed in the hot aqueous acid system.
It would therefore be highly desirable to have a method for improving the dispersion characteristic of diatomite and other siliceous filter aids in the "wet process" orthophosphoric acid production systems.